A vacuum-actuated tool for handling electronic workpieces is described in U.S. Pat. No. 5,169,192 to Allison et al. The tool includes a hand-held wand and a replaceable pickup tip that is press fit to the wand. The tip includes a vacuum pocket that is brought into contact with the electronic workpiece, allowing a workpiece to be carried under vacuum pressure. Typically, a wand includes a valve mechanism for selectively blocking an air evacuation path through the wand. Blocking the air evacuation path releases the electronic workpiece.
A concern in the handling of an electronic article, such as a semiconductor wafer, an integrated circuit chip or an integrated circuit package, is the damage that may be caused by electrostatic discharge (ESD). Even under careful handling procedures, a person or device can develop a high static voltage. Discharge of the voltage through an integrated circuit will generate electrical currents in excess of currents tolerable by the circuit. An ESD pulse can cause gate oxide breakdown or short circuiting of adjacent conductive signal traces within a circuit.
Structures for reducing the potential of circuit damage caused by ESD are known. The wand may be formed of an electrically conductive material, while the pickup tip includes forward and rearward conductive regions that are joined by a resistor. In one embodiment of this approach, the resistor is embedded into the pickup tip, so that the tip has the appearance of a conventional pickup tip. Typically, the wand is connected to ground potential. The embedded resistor provides an electrical path between the conductive wand and the forward portion of the tip. The resistive path limits the current between the wand and the electronic workpiece to be handled. However, an embedded resistor adds to the manufacturing complexity and cost of a pickup tip. The additional cost to an end-user is significant, since a number of tips are used during the life of the wand. Moreover, there may be a practical limit to the resistance of the embedded resistor, e.g., 10.sup.6 ohms.
Another option is to form the wand and the pickup tips of conductive materials and to connect the wand to ground potential. A wire may be connected from the wand to a ground point. A load resistor is wired in series to the electrical ground, thereby limiting the current of a static discharge. A problem with such a ground circuit is that a voltage pulse between a person handling the wand and the electronic workpiece being handled may cause circuit damage before current flow through the resistive ground circuit is able to dissipate the charge.
An attempt has been made to form a pickup tip of a resistive material. Because such a tip is a unitary structure, the manufacturing complexity and cost is less than that of a tip having an embedded resistor. While the resistive tip was found to work well for initial applications of ESD pulses, the resistance of the tip was reduced upon encountering an ESD pulse. Thus, the tips had to be replaced more often than conventional pickup tips.
It is an object of the present invention to provide an article-handling tool which achieves a more reliable controlled dissipation of static electricity.